I dont see how thrash metal would work with drop c tunings or even D for that matter. Thrash has a very distinct sound that can only be in standard tuning.
A classic death metal tuning would be tuning guitar a whole step down. DGCfad.
Thrash metal is on standard EADGBE or half step down. The farthest you can go is drop D.
For NWOBHM style its pretty standard.
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